US2863765A

US2863765A - Pure chromium

Info

Publication number: US2863765A
Application number: US648888A
Authority: US
Inventors: Reginald S Dean; Frank X Mccawley
Original assignee: Chicago Dev Corp
Current assignee: Chicago Dev Corp
Priority date: 1957-03-27
Filing date: 1957-03-27
Publication date: 1958-12-09
Anticipated expiration: 1975-12-09

Description

Dec. 9, 1958 R. s. DEAN ETAL PURE CHROMIUM Filed March 27, 1957 INVENTORS REGINALD 5: 05AM ITQANK X NCANLEY ATTORNEY PURE CHROMIUM Reginald S. Dean, Hyattsville, and Frank X. McCaWley, Cheverly, Md., assignors to Chicago Development Corporation, Riverdale, Md., a corporation of Delaware Application March 27,1957, Serial No. 648,888

1 Claim. (Cl. 75- 176) This invention relates to pure, chromium, articles made from it and methods for the preparation of same. It has for its aim the production of chromium by' electrorefining in the form of idiomorphic crystals and crystal intergrowths of hexagonal form, and the production from such chromium of alloys of chromium with other metals. It also has for its aim, the production of pure chromium in the form of ductile adherent coatings on metals.

In the known art, chromium has been produced by a variety of methods. Chromium made, for example, by electrolysis of aqueous solution and by magnesium reduction of chromium chloride contains oxygen which must be eliminated by other procedures as stated by Sully, Chromium, p. 55, Batterworths, London, p. 1954. The same authority states that chromium obtained from amalgam is very finely divided.

Pure chromium has been produced by dissociation of the iodide in the form of a coating on a wire. Crystals are not idiomorphic or macroscopically distinct.

Reduction of CrCl with hydrogen producedchromium powder of only 99% purity according to Maier in U. S. Bureau of Mines, Bull. 436,109 (1942,).

Electrolysis of fused salts has been disclosed by F. Krupp in U. K. Patent 197,887 (1922); Kroll et alt, U; S. Bureau of Mines Rep. 4752, December 1950, found, that metallic chromium produced in this Way contained 1.6% oxide.

A hexagonal form of chromium has been noted in plates but not in idiomorphic crystals. Further, the hexagonal structure is converted to the usual body centered cubic form by heating one hour at 150 according to Snavely, Trans-Electro Chem. Soc. 92,537 (1947) In one embodiment of our present invention, we produce chromium as macroscopic idiomorphic crystals of hexagonal form containing less than 01% oxygen; a group of such crystals is shown in Figure 1 magnified 2X; the hexagonal form is clearly shown by the outline of the crystals. The atomic arrangement of these crystals is shown by X-ray spectrometry to be body centered cubic;

the crystals are slowly attacked by dilute sulphuric and hydrochloric acid but are readily passivated by dilute nitric acid.

The chromium content of the crystals is more than 99.99% Cr. perature and become highly ductile at slightly elevated temperatures.

In a preferred process for the production of these chromium crystals, which we will describe in detail in examples, we pass a direct current from a comminuted crude chromium anode to an iron cathode in an electrolyte of sodium chloride having dissolved therein chromium chlorides and metallic sodium. The first step of such a process is the precipitation of small chromium crystals in a salt layer at the surface of the cathode. These crystals consolidate into a layer of consolidated chromium forming a ductile, highly protective'plate on the metal cathode. The large idiomorphic crystals and They are somewhat ductile at room temcrystal intergrowths of our invention are attached to the cathode by the salt layer containing dispersed fine chromium crystals.

The production of this plate is one of the objects of our invention, and the plated objects are one of the articles of our invention.

The plate consists of micro-crystals and is highly deformable and ductile. This layer of consolidated metal has an electrical resistance of 3.6 microhms cm. It is highly protective to-steel in salt water.

The chromium of our invention is particularly desirable for forming alloys with hexagonal metals such as titanium, zirconium, and cobalt by the processes of powder metallurgy.

Alloys made in this way are free from objectionable characteristics associated with chromium alloys of the known art. 1

The chromium crystals of our invention can also be used for the production of alloys by arc melting of consumable electrodes containing crystals of chromium of our invention and other high purity metals such as crystal intergrowths of pure titanium and zirconium.

Having now described our invention in its general terms, we will illustrate it by example.

We comminute this material to pass an 8 mesh screen, and place it in a foraminous nickel basket concentrically disposed around a steel rod in-an electrolytic cell provided with an argon atmosphere and having an electrolyte of molten NaCl. in which there is dissolved 5% Cr as chromium chloride, average valence 2.05 and .1% metallicsodium.

We prepare this electrolyte by melting a mixture of CrCl and NaCl, placing it in the cell described and passing a direct current from chromium in. the anode basket to the steel cell wall at 5 amperes until 1.2 faradays of current have been passed for each 52 grams of chromium present. We then place ferrochrome in the basket.

The approximate surface of the ferrochrorne in the anode basket is sq. ft. and the immersed surface of the initial cathode rod is .5 sq. ft.

We pass a direct current of 100 amperes for 3 hours at 800 C. 4

The cathode rod with adhering material is then removed from the bath and cooled, without access to air, 290 grams of macroscopic chromium crystals having a hexagonal form are removed from the cathode and washed with 1% HCl to remove salt. After drying these crystals had a hardness of 126 D. P. H. and were cold ductile.

They had less than .0l% oxygen and no other detectable impurities. The composition of the bath was not changed by the passage of current.

The cathode rod from which the crystals were removed was coated with a layer of salt 10 mils thick containing dispersed fine chromium crystals. This layer was scrubbed off and the rod was found to be plated with a layer 3 mils thick of ductile chromium in a continuous nonporous plate protective to steel in salt water.

The electrical resistance of the ductile chromium layer was 3.6 microhms cm. at room temperature.

Both the crystals and the plate show on analysis less than .0l% oxygen and no other determinable impurities.

3 Example 11 In this example, we proceed as in Example I, except as follows, the electrolyte is 65% SrCl 35% NaCl in which is dissolved 8.9% Cr as chloride having an average valence of 2.16 and .6% dissolved alkalinous metal; the temperature was 600 C.; the cathode is an alumium rod.

The results after 3 hours passage of current are identical with those of Example I including the formation on the aluminum rod of a non-porous ductile plate.

Example 111 In this example, We take crystals of pure chromium prepared in accordance with Example I and mix them with pure comminuted electrolytic cobalt in the proportion of 65% by weight, cobalt 35% by weight chromium. We compact this mixture at 50 tons per square inch pressure in a steel die. We then heat the so-formed compact to 950 C. in a vacuum of .01 micron for 8 hours. Th resulting alloy when quenched from 950 C. is entirely the hexagonal beta phase of the chromium cobalt system as described by Elsea, Westerman and Manning, Trans. A. I. M. E., 180,579 (1949). The alloy is malleable at room temperature and has a hardness of 300 Brinell. The quenched alloy can be hardened by heating to 600 C. After 3 hours at this temperature, it has a hardness of 500 Brinell without brittleness.

Example I V In this example, we take titanium crystal intergrowths having an oxygen content of less than .01% and mix these with 9% of chromium crystals produced in accordance with Example I. We compact these crystals into a consumable electrode and melt the electrode in an arc furnace having a water-cooled hearth at an argon pressure of .1 micron. The resulting ingot is heated in pure argon to 1000 C. and rapidly cooled.

The resulting beta titanium alloy is ductile and has a hardness of 250 Brinell. On heating to 600 C. for 1 hour, it is hardened to 500 Brinell without becoming brittle.

Example V In this example, we take crude chromium in pulverulent form and mix it with CrCl in proportions to form TiCl and add this to NaCl in such proportion that the total Cr content is 10% by weight. We heat this mixture to 800 C. and place it in a cell as described in Example I with a comminuted chromium anode and pass a current of amperes for 3 hours. The resulting bath analyzes 4 9.8 total soluble chromium; average valence determined by ferric sulphate solution 2.03; sodium by hydrogen evolution in acidified ferric sulphate solution 0.5%.

We then place a molybdenum cathode in the cell having an immersed surface of 2 square feet. The chromium in the anode basket has a surface of 1000 square feet. We pass a current of 500 amperes for 3 hours at 800 C.

The molybdenum cathode is then removed from the bath and cooled in argon. 1410 grams of macroscopic hexagonal chromium crystals are removed from the cathode.

The crude chromium in the anode analyzed 1.6% oxygen, 3% iron, balance substantially chromium. The crystals obtained from the cathode after washing analyzed less than .01% O and less than .01% Fe with no determinable amount of other impurities.

What is claimed is:

As an article of manufacture, macroscopic idiomorphic crystal intergrowths of chromium hexagonal as formed at above 750 C. and containing as formed less than 0.01% O and substantially no other impurity, said crystal intergrowths being pseudomorphs after the hexagonal form but having the crystal structure of alpha chromium when cooled to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,163,354 Schmidt et a1 June 20, 1939 2,446,996 Bouton et al Aug. 17, 1948 2,631,936 Kuhlmann Mar. 17, 1953 2,656,269 Dunn et al Oct. 20, 1953 2,658,266 Du Rose Nov. 10, 1953 2,702,239 Gilbert et a1 Feb. 15, 1955 2,752,303 Cooper June 26, 1956 2,780,545 Blank et a1 Feb. 5, 1957 2,782,114 Preston Feb. 19, 1957 2,789,943 Kittelberger Apr. 23, 1957 OTHER REFERENCES Institute of Metals, Journal, vol. 80, 1951-52; pages 112-114.

Institute of Metals, Journal, vol. 83, 1954-; pages 121-125.

Methalloberllache, 1953 (A)7, (10), -148. Abstracted on page 347, Institute of Metals Metallurgical Abstracts, vol. 22, Sept. 1954-Aug. 1955. Metallographic Investigation on Electrodeposited Chromium, Koch et a1.